MySQL - 将行作为列和列放置为行？

Question

如何将行转换为MySQL中的列？

我已经在StackOverflow上阅读了关于这个主题的一些问题，但我仍然不知道如何实现我想要的结果。我也试过一些没有成功的代码。

以下示例有望使我想要的结果清晰。我创建了一个名为“summary_kcal”的视图，该视图按用户ID分组。它的组成如下：

+---------+------+------+------+------+------+------+------+
| userid  |  LU  |  MA  |  ME  |  GIO |  VE  |  SA  | DO   | 
+---------+------+------+------+------+------+------+------+
| 1       | 1000 | 2000 | 1000 | 1500 | 2000 | 1000 | 1500 |
+---------+------+------+------+------+------+------+------+

我想将其转换为：

+---------+---------+
| userid  | 1       |
+---------+---------+
| LU      | 1000    | 
+---------+---------+
| MA      | 2000    | 
+---------+---------+
| ME      | 1000    |
+---------+---------+
| GIO     | 1500    |
+---------+---------+
| VE      | 2000    |
+---------+---------+
| SA      | 1000    |
+---------+---------+
| DO      | 1500    |
+---------+---------+

Answer 1

你通常最好做这种转换客户端，但如果你真的需要在数据库中做到这一点....

SELECT "LU" AS userid, `LU` AS `1` WHERE userid = 1
UNION
SELECT "MA" AS userid, `MA` AS `1` WHERE userid = 1
UNION
SELECT "ME" AS userid, `ME` AS `1` WHERE userid = 1
UNION
SELECT "GIO" AS userid, `GIO` AS `1` WHERE userid = 1
UNION
SELECT "VE" AS userid, `VE` AS `1` WHERE userid = 1
UNION
SELECT "SA" AS userid, `SA` AS `1` WHERE userid = 1
UNION
SELECT "DO" AS userid, `DO` AS `1` WHERE userid = 1

注意：您实际上只需要第一个查询中的别名，只是为了清晰和一致而包含。

如果你想要多个列，每个用户行一个列，这就变得难以管理，因为每个XX AS # WHERE userid = #最终都需要成为一个单独的子查询（或者是一个带有条件聚合的大型难以处理的GROUP CONCAT查询）。